feat(phase285): Complete weak reference implementation (VM + LLVM harness)

Phase 285LLVM-1.1 to 1.4 + weak reference infrastructure:

**LLVM Harness** (Phase 285LLVM-1.x):
- 285LLVM-1.1: User Box registration & debug output
- 285LLVM-1.2: WeakRef basic operations (identity deferred)
- 285LLVM-1.3: InstanceBox field access (getField/setField)
- 285LLVM-1.4: print Handle resolution (type tag propagation)

**VM Runtime** (nyash_kernel):
- FFI functions: nyrt_weak_new, nyrt_weak_to_strong, nyrt_weak_drop
  (crates/nyash_kernel/src/lib.rs: +209 lines)
- WeakRef plugin invoke support
  (crates/nyash_kernel/src/plugin/invoke.rs: +250 lines)
- weak_handles.rs: WeakRef handle registry (NEW)

**LLVM Python Backend**:
- WeakRef instruction lowering (weak.py: NEW)
- Entry point integration (entry.py: +93 lines)
- Instruction lowering (instruction_lower.py: +13 lines)
- LLVM harness runner script (tools/run_llvm_harness.sh: NEW)

**MIR & Runtime**:
- WeakRef emission & validation
- MIR JSON export for weak instructions
- Environment variable support (NYASH_WEAK_*, HAKO_WEAK_*)

**Documentation**:
- CLAUDE.md: Phase 285 completion notes
- LANGUAGE_REFERENCE_2025.md: Weak reference syntax
- 10-Now.md & 30-Backlog.md: Phase 285 status updates

Total: +864 lines, 24 files changed

SSOT: docs/reference/language/lifecycle.md
Related: Phase 285W-Syntax-0, Phase 285W-Syntax-0.1

src/runtime/mod.rs

@@ -31,6 +31,7 @@ pub mod unified_registry; // Deprecation warnings with warn-once guards
 pub mod extern_registry; // ExternCall (env.*) 登録・診断用レジストリ
 pub mod host_api; // C ABI: plugins -> host 逆呼び出しAPI（TLSでVMに橋渡し）
 pub mod host_handles; // C ABI(TLV) 向け HostHandle レジストリ（ユーザー/内蔵Box受け渡し）
+pub mod weak_handles; // Phase 285LLVM-1: WeakRef Handle レジストリ（bit 63 = 1）
 pub mod modules_registry;
 pub mod type_box_abi; // Phase 12: Nyash ABI (vtable) 雛形
 pub mod type_meta;